Vehicle power supply unit

ABSTRACT

A vehicle power supply unit is provided, which includes a part made of degradable plastic, which may be a battery case of a secondary battery, and may be a separator of a fuel cell when the degradable plastic is made conductive. A condition in which the degradable plastic is decomposed is different from a condition in which it is actually used in a vehicle.

INCORPORATION BY REFERENCE

This is a division of application Ser. No. 10/387,844 filed 14 Mar.2003, which claims priority to Japanese Patent Application No.2002-078821 filed on Mar. 20, 2002, the contents of which areincorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a vehicle power supply unit including a plasticpart.

2. Description of Related Art

An insulating material mainly made of plastic is conventionally used forforming a part used in a vehicle power supply unit for an electricvehicle or the like. Also, some plastics are made conductive and used asconductive materials. For example, Japanese Laid-Open Patent PublicationNo. 2000-17179 discloses a fuel cell including a separator made ofconductive plastic (resin). Also, a secondary battery whose battery caseis made of plastic is known.

As is widely known, various environmental measures are taken fordisposing of regular vehicle parts. Likewise, when disposing of aplastic part for use in electric vehicles, such as a plastic separatorof a fuel cell, some environmental measures need to be taken. However,when disposing of such a plastic part of a vehicle power supply unit,the following problem is concerned. That is, when plastics (wasteplastics) are buried in the ground together with other waste materials,they remain undecomposed semi-permanently in the ground. Also, when theplastics are burnt using an incinerator, or the like, the useful life ofthe incinerator reduces owing to high energy required for burning theplastics, or toxic substances are released into the air, thus damagingthe environment.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a vehicle powersupply unit which does not cause an environmental problem when and afterbeing disposed of.

A vehicle power supply unit according to a first aspect of the inventionincludes a part made of degradable plastic. Made of degradable plastic,the part is decomposed into carbon dioxides and water by microorganisms,light, or the like. Therefore, it resolves the problem that a plasticpart is not decomposed after being disposed of and thus remains as awaste semi-permanently.

According to a further aspect of the invention, it is preferable thatthe part made of degradable plastic is a separator of a fuel cell. Whenthe separator of the fuel cell is formed of degradable plastic, itprevents an environmental problem which may otherwise be caused when aconventional separator formed of non-degradable material is disposed of.

According to a further aspect of the invention, it is preferable thatthe part formed of degradable plastic is a battery case of a secondarybattery. When the battery case of the secondary battery is formed ofdegradable plastic, it achieves the same effects and advantages as thoseobtained when the separator of the fuel cell is formed of degradableplastic.

According to a further aspect of the invention, it is preferable thatthe degradable plastic has a conductivity. When the degradable plastichas a conductivity, it may be used for forming a separator of a fuelcell.

According to a further aspect of the invention, it is preferable that acondition in which the degradable plastic is decomposed is differentfrom a condition in which the degradable plastic is used in a vehicle.Thus, the degradable plastic is not decomposed into pieces during usage.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and/or further objects, features and advantages of theinvention will become more apparent from the following description ofpreferred embodiments with reference to the accompanying drawings, inwhich like numerals are used to represent like elements and wherein:

FIG. 1 is an exploded perspective view showing a cell of a fuel cellsystem as a power supply unit according to one embodiment of theinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereafter, a vehicle power supply unit according to a preferredembodiment of the invention will be explained with reference to FIG. 1.The vehicle power supply unit includes a part which is formed ofdegradable plastic. Some examples of the degradable plastic are listedbelow.

1. Photodegradable Plastic

The photodegradable plastic is one type of plastic which is decomposedby lights (mainly ultraviolet lights). More specifically, thephotodegradable plastic is decomposed into a low-molecular-weightcompound by lights. In some cases, it is also decomposed into alow-molecule-weight compound by oxygen, water, or the like. Then, thelow-molecule-weight compound is completely decomposed into carbondioxide gas and water by microorganisms, and is thus returned to thenature. That is, the photodegradable plastic includes a certain chemicalcompound (e.g., a carbonyl group) which is decomposed by lights.Meanwhile, photodegradable plastic is classified into a photosensitivefunctional group introduction type in which the certain compound isintroduced into a polymer, and a photosensitive reagent addition type inwhich a certain additive is used.

Examples of photodegradable plastic of the photosensitive functionalgroup introduction type are an ethylene-carbon monoxide copolymer and avinyl ketone copolymer (e.g., methyl vinyl ketone, ethyl vinyl ketone).They are decomposed in approximately 20 to 24 hours when exposed toultraviolet lights, and in approximately in 2 to 8 weeks when exposed tolights in the natural environment.

In the meantime, aromatic ketone, such as benzophenone, acetophenone,and anthraquinone, may be used as a photosensitizer used for obtainingthe photodegradable plastic of photosensitive reagent addition type.

2. Biodegradable Plastic

The biodegradable plastic is a plastic which is decomposed bymicroorganisms and is thus returned to the ecological cycle. Namely, itis eventually decomposed into carbon dioxide gas and water. Thebiodegradable plastic can be broadly classified into the following threegroups A, B and C.

A) High polymer produced by microorganisms

For example, a hydroxybutyrate-based polyester may be used.

B) Natural high polymer material such as starch, cellulose and chitin

This is a biodegradable plastic that is mainly made of natural highpolymer material, such as starch extracted from a crop, such as a corn,a potato, a wheat, and a sugar cane, cellulose extracted from a plant,chitin extracted from a shell of crab, shrimp, or the like.

C) Degradable synthetic plastic;

For example, polycaprolactone may be used.

When the part used in the vehicle power supply unit is formed of suchdegradable plastic, the part is returned to the nature after beingdisposed of. Thus, the environmental problems described above, which mayotherwise be caused when disposing of the part, can be prevented. In thecase of a vehicle power supply unit, the degradable plastic can be usedfor forming a separator of a fuel cell, a battery case of the secondarybattery, and so on.

A fuel cell system 10 is constituted of a plurality of cells stackedtherein. As shown in FIG. 1, each cell of the fuel cell system 10includes a permeable membrane/electrode assembly (MEA) 11, diffusionlayers 12, 13 as porous carbon membranes located in both sides of themembrane/electrode assembly 11, and separators 14, 15. Further, sealframes 16, 17 are interposed. Generally, the separators 14, 15 areprovided by a plate made of carbon, metal or conductive resin, withpassages formed thereon for delivering gas or coolant. When forming aseparator with a degradable plastic, it is therefore necessary to makethe degradable plastic conductive.

In the following, four examples of a method for obtaining a degradableplastic having a conductivity are described.

-   1. Mixing a surface-active agent or an antistatic agent into the    degradable plastic, or applying the surface-active agent or the    antistatic agent to the surface of the degradable plastic.-   2. Mixing a conductivity imparting agent (this is also referred to    as a conductive filler) into the degradable plastic.

For example, conductive metal powder or conductive metal fiber such asflaky graphite, carbon-based powder, carbon-based fiber, silver, copperand nickel may be used.

-   3. Forming a metallized film on the degradable plastic molded    product.    For example, the film may be made of oxidized tin.-   4. Newly designing the degradable plastic.    A conductive high polymer may be synthesized by changing the    chemical structure thereof.

When the part in the vehicle power supply units is formed of thepolymeric material of the various degradable plastics described above,existing molding devices may be used, such as an extruder, an injectionmolding device, and a blow molding device which are generally used formolding thermoplastic that is produced in the petrochemical industry.Also, when a separator of a fuel cell is formed of the conductive anddegradable plastic, the polymeric material of the respective degradableplastics and the conductivity imparting agent (e.g., flaky graphite,carbon powder, carbon fiber) are mixed by means of a twin-screw extruderor the like, and the resultant mixture is then introduced into themolding device to be formed into a desired shape.

Hereinafter, one example of a method for forming a separator of a fuelcell from conductive and degradable plastic will be described. In thismethod, the separator is formed using three materials 1, 2, and 3according to a procedure 4.

1. Degradable Plastic

Starch extracted from a corn is used as the biodegradable plastic whichis decomposed by microorganisms.

2. Conductivity Imparting Agent

Flaky graphite (carbon black) is used as the conductivity impartingagent. Here, flaky graphite having a grain size smaller than severalmicrometers is preferably used. While the amount of the conductivityimparting agent to be added is set to 20 weight percent or lower in thisexample, it is not necessarily set to 20 percent or lower as long as adesired conductivity can be imparted to the separator. Nevertheless, itis preferable, in terms of the disposability of the separator, that theamount of the conductivity imparting agent is made as small as possible,since it remains even after the plastic has been decomposed.

3. Water

Water is added to when mixing the degradable plastic and theconductivity imparting agent so that the mixture has a liquid form whichis suitable for use in conventional molding devices, such as anextruder, injection molding device, and blow molding device, which aregenerally used for molding thermoplastic.

4. Manufacturing Procedure

Starch extracted from a corn, carbon black, and water are mixed by thetwo-screw extruder at a temperature of 100° C. under a pressure of 30kgf/cm2. The resultant mixture having a liquid form is then deliveredinto the molding device (e.g., extruder, injection molding device, blowmolding device) to form a separator.

Next, a fuel cell including the separator manufactured as describedabove was made and a power generation test was performed with the fuelcell. The result of the test indicates that the separator has a desiredconductivity and a necessary level of operation stability and mechanicalstrength under the operating temperature thereof. Subsequently, the fuelcell was disassembled and the separator was taken out. The separator wasthen placed in a condition that causes decomposition thereof(hereinafter will be referred to as “a decomposing condition” whereappropriate), and it was confirmed, as a result, that the separator isdecomposed under the decomposing condition. Meanwhile, this decomposingcondition may be created by using, for example, activated sludge, soil,or certain enzymes (e.g., amylase and lipase secreted from cell bodiescan be effectively used as enzymes for decomposing starch) andmicroorganisms. Among these options, when enzymes and microorganisms areused, the decomposition of the separator is completed within arelatively short time (several days to several weeks). Although carbonblack remains even after the separator has been completely decomposed,it may be reused.

In the case where such a degradable plastic part is used in a vehiclepower supply unit, not to mention, it is necessary to arrange the powersupply unit in a vehicle so that the part is not placed in theabove-described decomposing condition. Namely, the condition in whichthe degradable plastic part is decomposed is required to be differentfrom the condition in which it is actually used in the vehicle. In thecase of a part formed of photodegradable plastic, for example, it isnecessary to dispose the power supply unit within a casing, or the like,so that the part is not exposed to lights. Also, in the case of a partmade of biodegradable plastic, it is necessary to dispose the powersupply unit within a casing, or the like, so that the part is isolatedfrom microorganisms. Thus, since a fuel cell stack is normally disposedin a casing, the above requirement is fulfilled.

According to the vehicle power supply unit of the above-illustratedembodiment, a degradable plastic is used, which is decomposed intocarbon dioxide and water by lights, microorganisms, and so on.Therefore, it solves the problem that plastic is not decomposedsemi-permanently after being disposed of.

Also, according to the vehicle power supply unit of theabove-illustrated embodiment, a separator of a fuel cell is formed ofsuch a degradable plastic, which prevents the environmental problemwhich may be caused after a separator made of non-degradable plastic isdisposed of. Moreover, while the degradable plastic is used for formingthe separator of the fuel cell in the above-illustrated embodiment, itmay also be used for forming a battery case of a secondary battery. Inthis case, too, the same effects and advantages can be obtained.

Also, according to the vehicle power supply unit of theabove-illustrated embodiment, since the degradable plastic is madeconductive, it can be used for forming a separator of a fuel cell, orthe like, which is required to be conductive.

Also, according to the vehicle power supply unit of theabove-illustrated embodiment, the condition in which the degradableplastic is decomposed differs from the condition in which it is actuallyused in the vehicle. Therefore, there is no possibility that thedegradable plastic is decomposed into pieces during usage.

1. A vehicle power supply unit including a plastic part, wherein theplastic part is a battery case of a secondary battery and the vehiclepower supply unit is a fuel cell; the part comprises a degradableplastic which is decomposable by light or by microorganism and anelectrical conductivity imparting agent, wherein the electricalconductivity imparting agent comprises 20 percent or less of a weight ofthe part.
 2. The vehicle power supply unit according to claim 1, whereinthe vehicle power supply unit is covered by or contained in a powersupply unit case.